The present invention relates to the field of boron trifluoride and sulphuric acid and relates more particularly to the conversion of industrial effluents of boron trifluoride hydrate to boron trifluoride and sulphuric acid.
Boron trifluoride is a gas which is principally used in industry as a catalyst in a large number of reactions: polymerization, esterification, alkylation and isomerization. Customarily, after use, the boron trifluoride is treated with an aqueous sodium hydroxide solution and the resulting effluent, made up of derivatives containing fluorine and borine, is discarded.
In order to avoid these fluorine- and borine-containing wastes, the boron trifluoride can be recovered after use, at the end of reaction, in the form of solutions of BF3 hydrate which are obtained on the one hand by bringing the BF3 into contact with water and, on the other hand, by washing the organic compounds formed in the reaction catalysed by BF3 with water (see, for example, the patent EP 364 815).
Owing to the presence ofxe2x80x94essentially organicxe2x80x94impurities, the solutions of boron trifluoride hydrate are generally coloured to a greater or lesser extent, it being possible for their content of organic carbon to range from several ppm to several thousand ppm (customary values: from approximately 10 to 1000 ppm).
The patent U.S. Pat. No. 5,536,484 describes a process for upgrading technical BF3 hydrates in the form of aqueous solutions of tetrafluoroboric acid corresponding to commercial specifications. However, the final step of decolouring requires the use of an active carbon which must be destroyed after use.
Another means of upgrading a technical BF3 hydrate is set out in the patent EP 364 815 and consists of regenerating the boron trifluoride by treating the technical hydrate with sulphuric acid, oleum or SO3. Unfortunately, the sulphuric acid resulting from this operation has a yellow to black coloration, making it unsuitable for use without prior purification treatment.
On the other hand, organic substances and carbon dioxide are unacceptable in a commercial BF3 intended for applications in catalysis. The current commercial product must contain less than 10 mg of organic carbon and less than 5 mg of CO2 per kg of BF3.